src: fix abort when taking a heap snapshot

Remove an erroneous CHECK that asserted the persistent object's internal
field pointer still pointed to a valid object.  If ClearWrap() has been
called, the field pointer equals nullptr and that is expected behavior.

PR-URL: https://github.com/nodejs/node/pull/18898
Fixes: https://github.com/nodejs/node/issues/18256


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>
Reviewed-By: Ruben Bridgewater <ruben@bridgewater.de>
Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Matheus Marchini <matheus@sthima.com>